The search for a small float plane with four people on board that went missing in western Alaska failed to find any signs of a crash or pick up any signals from the plane’s emergency transmitter by late Sunday, the Anchorage Daily News reported.

The search — coordinated by the Park Service and utilizing teams from the U.S. Coast Guard, the Alaska Air National Guard, the Park Service, U.S. Fish and Wildlife Service, Alaska State Troopers and a private air service — was hampered by poor visibility and was expected to wrap up by 10:30pm local time.

Three Park Service rangers and a pilot were on board the de Havilland Beaver, which was reported missing Saturday afternoon after it did not arrive at King Salmon Airport as expected. The plane’s flight, from Swikshak Bay, Alaska, to King Salmon, normally takes less than an hour, the Coast Guard said in a statement. It took off about 1:45pm local time with a pilot who was “very familiar with the area and had flown the route many times,” according to Katmai National Park Superintendent Ralph Moore.

A second plane carrying two additional park employees that left 15 minutes after the missing plane completed its journey in about an hour, the Park Service said, but was forced by worsening weather conditions to drop to an altitude of just 500 feet (152 meters) and follow a river drainage. Its pilot reportedly noticed nothing amiss but said weather forced the plane to fly a different route to avoid mountainous terrain.

The missing rangers — named by the National Park Service as 26-year-old Mason McLeod and brothers Neal Spradlin, 28, and Seth Spradlin, 20 — worked in the maintenance department of Katmai National Park and had waited in Swikshak Bay for days for a delivery of construction materials that was delayed by weather. The name of the pilot of the Branch River Air Service plane was not released.

Moore said it was possible the plane was forced to make an emergency landing and that its passengers were waiting out the storm in a cove, adding that the emergency signal could be blocked by the mountains.

Alaska was hit by four other plane crashes earlier this summer, which caused the deaths of 13 people.